PETROGRAPHY AND GEOCHEMISTRY OF FELSIC DIKES FROM THE EASTERN MARGIN OF THE MT. WALDO PLUTON, COASTAL MAINE

The Mt Waldo pluton (371± 2 Ma, Stewart et al., 1995) crops out over an area of 160 km2 at the north end of Penobscot Bay, Maine. Petrographic and geochemical evidence suggests a definite role for magma mixing in the evolution of this pluton (Gibson and Lux, 1999). At the eastern margin of the Mt Waldo pluton a number of felsic dikes intrude the country rocks, i.e. the Ordovician age Penobscot formation. These are observed in a number of roadside exposures along Route 1, south of Bucksport, on the western side of Penobscot Bay. However, it is unclear whether these dikes are related to the intrusion of the Waldo granite or if they represent a different phase of felsic magmatism in the area.

The dikes are a distinctive light blue-gray color and contrast markedly with the black, rusty-weathered sulfidic schists. They have a fine-grained, equigranular texture and are extremely leucocratic (CI< 10). The dikes contain both muscovite and biotite. Tourmaline is common in many hand specimens. They range in thickness from 20cm to around 1M and have variable orientations. In some cases they are vertical with a general E – W trend, whereas others have sheet-like forms dipping at variable inclinations to the NE. Crosscutting relationships are not common between the two main forms, though evidence at one locality suggests that there may be more than one generation of intrusion.

There are a number of possibilities regarding the origin of these dikes. 1) They may be offshoots of the chilled marginal facies of the Waldo pluton. Even though there is no direct field evidence that the dikes originate from the Waldo this would be significant as they could represent the felsic end-member of the magma mixing series. 2) Alternatively they could be late stage aplites from the Waldo granite. Some Waldo aplites do contain minor amounts of tourmaline but only secondary muscovite. 3) Another source could be the two-mica Wallamatogus pluton (397±2 Ma, D.R. Lux, unpublished data) which outcrops just 2 – 3 km to the east across Penobscot Bay. Geochemical analyses of these various rock types should help us constrain more definitively the origin of these felsic dikes.